Scanning system with adjustable optical characteristcs

ABSTRACT

An optical scanner for scanning various optical indicia uses one or more tunable optical components for noise reduction purposes. The scanner may include a narrow band-pass tunable optical filter for passing selected wavelengths of incoming light. The scanner may also have a tunable light source, such as a tunable laser source, whereby the wavelength of the generated laser light is sensed and controlled by the scanner&#39;s electronic sub-system.

FIELD OF THE INVENTION

[0001] This invention relates to the field of optical scanners such asbar code scanners. More specifically, the invention relates to the useof tunable optical components for noise reduction within opticalscanners.

BACKGROUND OF THE INVENTION

[0002] Electro-optical scanners, such as bar code symbol scanners, arenow quite common. Typically, a bar code symbol comprises one or morerows of light and dark regions, typically in the form of rectangles. Thewidths of the dark regions, i.e., the bars, and/or the widths of thelight regions, i.e, the spaces between the bars, when partitioned intogroups, indicate encoded information to be read.

[0003] A bar code symbol reader illuminates the symbol and senses lightreflected from the coded regions to detect the widths and spacings ofthe coded regions and derive the encoded information. Bar code readingtype data input systems improve the efficiency and accuracy of datainput for a wide variety of applications. The ease of data input in suchsystems facilitates more frequent and detailed data input, for exampleto provide efficient inventories, tracking of work in progress, etc.

[0004] A variety of scanning systems are known. One particularlyadvantageous type of reader is an optical scanner which scans a beam oflight, such as a laser beam, across the symbols. Laser scanner systemsand components of the type exemplified by U.S. Pat. Nos. 4,387,297 and4,760,248—which are owned by the assignee of the instant invention andare incorporated by reference herein—have generally been designed toread indicia having parts of different light reflectivity, i.e., barcode symbols, particularly of the Universal Product Code (UPC) type, ata certain working range or reading distance from a hand-held orstationary scanner.

[0005]FIG. 1 illustrates an example of a prior art bar code symbolreader 10 implemented as a gun shaped device, having a pistol-grip typeof handle 53. A lightweight plastic housing 55 contains a light source46, a detector 58, optics 57, signal processing circuitry 63, aprogrammed microprocessor 40, and a power source or battery 62. An exitwindow 56 at the front end of the housing 55 allows an outgoing lightbeam 51 to exit and an incoming reflected light 52 to enter. A user aimsthe reader at a bar code symbol 70 from a position in which the reader10 is spaced from the symbol, i.e., not touching the symbol or movingacross the symbol.

[0006] As further depicted in FIG. 1, the optics may include a suitablelens 57 (or multiple lens system) to focus the scanned beam into ascanning spot at an appropriate reference plane. The light source 46,such as a semiconductor laser diode, introduces a light beam into anoptical axis of the lens 57, and other lenses or beam shaping structuresas needed. The beam is reflected from an oscillating mirror 59 which iscoupled to a scanning drive motor 60 energized when a trigger 54 ismanually pulled. The oscillation of the mirror 59 causes the outgoingbeam 51 to scan back and forth in a desired pattern.

[0007] A variety of mirror and motor configurations can be used to movethe beam in a desired scanning pattern. For example, U.S. Pat. No.4,251,798 discloses a rotating polygon having a planar mirror at eachside, each mirror tracing a scan line across the symbol. U.S. Pat. Nos.4,387,297 and 4,409,470 both employ a planar mirror which isrepetitively and reciprocally driven in alternate circumferentialdirections about a drive shaft on which the mirror is mounted. U.S. Pat.No. 4,816,660 discloses a multi-mirror construction composed of agenerally concave mirror portion and a generally planar mirror portion.The multi-mirror construction is repetitively reciprocally driven inalternative circumferential directions about a drive shaft on which themulti-mirror construction is mounted.

[0008] The light 52 reflected back by the symbol 70 passes back throughthe window 56 for transmission to the detector 58. In the exemplaryreader shown in FIG. 1, the reflected light reflects off of mirror 59,passes through an optical filter 47 and impinges on the light sensitivedetector 58. The filter is typically designed to have a band-passcharacteristics in order to pass the reflected (return) laser light andblock the light coming from other light sources. The detector 58produces an analog signal proportional to the intensity of the reflectedlight 52.

[0009] The signal processing circuitry includes a digitizer 63 mountedon a printed circuit board 61. The digitizer processes the analog signalfrom detector 58 to produce a pulse signal where the widths and spacingsbetween the pulses correspond to the widths of the bars and the spacingsbetween the bars. The digitizer serves as an edge detector or waveshaper circuit, and a threshold value set by the digitizer determineswhat points of the analog signal represent bar edges. The pulse signalfrom the digitizer 63 is applied to a decoder, typically incorporated inthe programmed microprocessor 40 which will also have associated programmemory and random access data memory. The microprocessor decoder 40first determines the pulse widths and spacings of the signal from thedigitizer. The decoder then analyzes the widths and spacings to find anddecode a legitimate bar code message. This includes analysis torecognize legitimate characters and sequences, as defined by theappropriate code standard. This may also include an initial recognitionof the particular standard to which the scanned symbol conforms. Thisrecognition of the standard is typically referred to asautodiscrimination.

[0010] To scan the symbol 70, the user aims the bar code reader 10 andoperates movable trigger switch 54 to activate the light source 46, thescanning motor 60 and the signal processing circuitry. If the scanninglight beam 51 is visible, the operator can see a scan pattern on thesurface on which the symbol appears and adjust aiming of the reader 10accordingly. If the light beam 51 produced by the source 46 ismarginally visible, an aiming light may be included. The aiming light,if needed, produces a visible-light spot which may be fixed, or scannedjust like the laser beam 51. The user employs this visible light to aimthe reader at the symbol before pulling the trigger.

[0011] The reader 10 may also function as a portable data collectionterminal. If so, the reader would include a keyboard 48 and a display49, such as described in the previously noted U.S. Pat. No. 4,409,470.

[0012] In electro-optical scanners of the type discussed above, thelaser source, the optics, the mirror structure, the drive to oscillatethe mirror structure, the photodetector, and the associated signalprocessing and decoding circuitry can all be packaged in a “scanningmodule”, which in turn is placed into the scanner's, or terminal'shousing.

[0013] One of the factors which can affect scanner's performance is it'ssignal-to-noise ratio. The signal-to-noise ratio can be separated intotwo components, optical and electrical. The optical signal-to-noiseratio depends upon the amount of the reflected diffused laser lightdetected by the sensor 58, verses the amount of parasitic lightimpinging upon the sensor 58 which comes from other light sources suchas an ambient light. In order to increase the optical signal-to-noiseratio, the band-pass filter is designed to transmit in the spectrum ofthe laser source and to block the light in other optical spectrums. Atypical bandwidth of the optical filter used in bar code scanners isapproximately 70 nanometers. In comparison, a typical bandwidth of alaser beam generated by a laser diode is on the order of few nanometers.The large difference between the filter bandwidth and the laser beambandwidth is due to a number of variables.

[0014] The optical filter has to accommodate laser wavelengthvariations, as well as the laser wavelength changes due to temperature.In addition sufficient bandwidth must be allocated for filter variationsand filter slope roll-off.

[0015] Therefore, a need exists for a scanner having very narrow opticalband-pass filter. This would increase the scanner's signal-to-noiseratio and improve the system's performance in high optical noiseenvironments.

[0016] The need also exists for a method of making a scanner having anarrow optical band-pass filter which would accommodate laser wavelengthvariations.

[0017] There is also a need for a scanner having a narrow band-passoptical filter which is capable of adjusting its filteringcharacteristics in order to track the laser wavelengths variations.

[0018] There is a further need for a scanner capable of controllinglaser wavelengths, thus allowing the use of a non-tunable or partlytunable narrow band-pass optical filter.

[0019] Solutions for the above objects of the invention are fullydisclosed in the drawings and the following pages of the specification.

SUMMARY OF THE INVENTION

[0020] These and other objects of the present invention are achieved inaccordance with the present invention by a barcode reader and a methodfor reading barcodes in accordance with the present invention.

[0021] The barcode reader in one embodiment comprises a light source forgenerating a barcode illuminating beam and a detector for detectingreturn light reflected from a barcode illuminated by the illuminatingbeam and producing a signal corresponding thereto. A tunable opticalfilter is disposed before the detector to filter the return light. Thetunable optical filter has an adjustable filtering characteristic thatis adjustable to maximize the signal-to-noise ratio of the detector andthereby the barcode reader.

[0022] The light source is preferably a monochromatic light source, andmost preferably, a laser light source. The tunable optical filter can bean interference type optical filter and preferably has a lightcollimator located in front of the filter to provide a collimated returnlight to the optical filter.

[0023] The filtering characteristic of the filter can be adjusted bychanging an angle of incidence of the return light with respect to theoptical filter. The optical filter is most preferably a band-passtunable optical filter with a band-pass filtering characteristic, and ina preferred embodiment, the interference type optical filter includesinterference coating layers preferably having a plurality of coatinglayers on one side of the optical filter.

[0024] In a preferred embodiment, the plurality of interference coatinglayers is separated into sets of layers, and one set of layers islocated on one side of the optical filter, and another set of layers islocated on the other side of the optical filter.

[0025] In one embodiment, the angle of incidence of return light ischanged by an adjuster, which can be either passive or active. Thepassive adjuster is preferably a temperature sensitive bimetallicspring, and a preferred embodiment of an active adjuster is a sensor forsensing the temperature of the light source and an actuator forpositioning the optical filter at an angle that increases thesignal-to-noise ratio of the barcode reader.

[0026] The active adjuster can further include a lookup table forcorrelating a set of angular positions of the optical filter to a set oftemperatures for the light source. The active adjuster can be an analogelectromechanical adjuster.

[0027] In another embodiment, the active adjuster can sense thewavelength of the laser beam, and an actuator positions the opticalfilter at an angle that increases the signal-to-noise ratio based on thesensed wavelength. This can also be carried out by using a lookup tablefor correlating a set of angular positions to a set of wavelengths.

[0028] The tunable optical filter can also be an LCD type opticalfilter, an acoustic type optical filter, or a diffractive type opticalfilter.

[0029] In another embodiment of the present invention, in addition tothe light source, detector and an optical filter, the reader has acontroller for adjusting the wavelength of the illuminating beam tomaximize the signal-to-noise ratio of the signal produced by thedetector. In this embodiment, the optical filter has a staticcharacteristic, whereas the wavelength of the light source is modulatedby varying the temperature or the current to the light source.Alternatively, both the current and the temperature to the light sourcecan be varied to adjust the wavelength of the beam.

[0030] The present invention also relates to methods for readingbarcodes which include the steps of adjusting the filteringcharacteristic of a tunable filter to maximize signal-to-noise ratio,adjusting the filtering characteristic of the optical filter by changingthe angle of incidence of the return light in order to increase thesignal ratio, or controlling the wavelength of the illuminating beam tomaximize the signal-to-noise ratio of the reader.

[0031] These and other features and advantages of the present inventionwill become more apparent from the detailed description of the inventiontaken with the attached drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 illustrates a prior art optical scanner;

[0033]FIG. 2 illustrates filtering characteristics of a tunableband-pass filter;

[0034]FIG. 3 illustrates an optical scanner having a tunable band-passoptical filter according to the preferred embodiment of the invention;

[0035]FIG. 4 illustrates a shift in special transmission characteristicof an interference filter as a function of an angle of incidence of theincoming light;

[0036]FIG. 5 illustrates an optical subsystem utilizing a tunableinterference band-pass filter;

[0037]FIG. 6 illustrates an optical scanner having a tunable lasersource according to the preferred embodiment of the invention;

[0038]FIG. 7 illustrates an electro-optical subsystem for controllingthe wavelength of a laser source.

DETAILED DESCRIPTION OF THE INVENTION

[0039] Referring now to FIG. 2, the filtering characteristics of atunable band-pass filter in accordance with the present invention isshown. For barcode scanners, the bandwidth of the optical band-passfilter is a critical parameter to limit the amount of ambient noisegetting into the electric optical system, which significantly impactsscanner performance. For conventional optical band-pass filters, thedetermination of the bandwidth will be based on the laser wavelengthvariation of the laser diode, the laser wavelength shift due totemperature range, the numerical aperture of collection optics and thetolerances of either front or back slope of the filter. To cover all ofthese major variables, the bandwidth of the optical band-pass filter isvery difficult to be reduced.

[0040] Since the laser diode is a very good coherent light source, thismeans that the optical spectrum of the laser wavelength is very narrow,usually less than a nanometer. The optical band-pass filter, accordingto the present invention, has dielectric coatings coated on both sidesof the optical transparent substrate, for example, float glass, tocompose the optical spectrum bandwidth. The bandwidth is measured by thelaser coherency plus the numerical aperture of collection optics. Thespectrum location of such bandwidth can be tuned to cover the actuallaser wavelength. Thus, as shown in FIG. 2, the filtering characteristic70 of the band-pass filter can be tuned to vary from a center wavelength74 to a characteristic 72 having a center wavelength 76.

[0041] The tuning method can be either mechanical or optical. Forexample, changing the incident angle of the laser beam to the opticalband-pass filter can shift the bandwidth spectrum location. This shiftis illustrated in FIG. 4, which shows the characteristic of aninterference filter as a function of an angle of incidence of theincoming light. Reference numeral 90 points to a location on the filtertransmission curve where a change if 40 degrees in the angle ofincidence produces approximately 60% reduction in the filtertransmission characteristic at a particular wavelength.

[0042] Alternatively, the dielectric coding layer structure materialscan be changed to achieve the same optical spectrum temperature shiftfor the optical band-pass filter as for the laser wavelength of laserdiode. This shift is shown in FIG. 2.

[0043]FIG. 3 illustrates the scanner having the tunable band-passoptical filter 80 controlled by a controller 82 for changing the angleof the filter. The tunable band-pass filter of FIG. 3 is shown in moredetail in FIG. 5.

[0044] As shown therein, a collimator 96 is preferably placed in frontof the interference filter 81 to provide a collimated return light tothe optical filter. The collimated light 95 is passed to the filter 81,which can be shifted by an angle a from the position 98 to the position97, which shifts the filtering characteristic of the filter and thusmaximizes the light returned to detector 58, which produces anelectrical signal 102 having a maximized signal to noise ratio.

[0045] The filter 81 shown in FIG. 5 preferably has layers ofinterference coating on at least one side thereof. The adapter 82changes the angle of incidence of the filter either passively oractively. For example, the adapter 82 can be a temperature sensitivebimetallic spring and passively adjusts the angle. Adapter 82 can be anactive adjuster, and for example, be a sensor for sensing thetemperature of the light source and an actuator for moving the filterbetween the positions shown. The sensor can also sense the wavelength ofthe laser beam and alter the position of the filter as shown. Theadapter 82 can also include a lookup table for correlating angularpositions to temperatures or wavelengths as described above.

[0046]FIG. 6 shows an alternative embodiment of the present inventionwherein the light source 120 is tunable and has an apparatus forcontrolling the wavelength of the light source.

[0047]FIG. 7 illustrates the subsystem 130 for controlling thewavelength of the laser source.

[0048] In the system shown in FIG. 7, the wavelength of the laser 120 ismodified by a combination of temperature and current modulation bymaximizing the output of a first monitoring photodiode 131, whichreceives its light through an optical filter 132, which is identical tothe filter 47 placed before the detector 58. The maximizing of theoutput of photodiode 131 is subject to maintaining a safe level ofoutput radiation through a second monitoring photodiode 133. Drivecircuitry 134 receives the signals from diodes 131 and 133 and controlsthe current to laser 120 and controls the temperature of the laser via aheater/cooler 135 in contact with the laser 120.

[0049] It is understood that the embodiments described hereinabove aremerely illustrative and are not intended to limit the scope of theinvention. It is realized that various changes, alterations,rearrangements and modifications can be made by those skilled in the artwithout substantially departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. A bar code reader comprising: a) a light sourcefor generating a bar code illuminating beam; b) a tunable optical filterfor passing a return light reflected from an illuminated bar code, saidtunable optical filter having an adjustable filtering characteristic;and c) a detector for detecting the return light transmitted throughsaid optical filter; whereby said filtering characteristic is adjustedto maximize a signal-to-noise ratio of said bar code reader.
 2. The barcode reader of claim 1, wherein said light source is a monochromaticlight source.
 3. The bar code reader of claim 2, wherein said lightsource is laser light source and wherein said illuminating beam is alaser beam.
 4. The bar code reader of claim 1, wherein said tunableoptical filter is an interference-type optical filter.
 5. The bar codereader of claim 4, further including a light collimator located in frontof said optical filter and in a path of the return light for modifyingthe return light and for providing a collimated return light to saidoptical filter.
 6. The bar code reader of claim 4, wherein saidfiltering characteristic is adjusted by changing an angle of incidenceof the return light with respect to said optical filter.
 7. The bar codereader of claim 6, wherein said tunable optical filter is a band-passtunable optical filter and said filtering characteristic is a band-passfiltering characteristic.
 8. The bar code reader of claim 7, whereinsaid interference-type optical filter includes a plurality ofinterference-coating layers.
 9. The bar code reader of claim 8, whereinsaid plurality of interference-coating layers is located on a first sideof said optical filter.
 10. The bar code reader of claim 8, wherein saidplurality of interference-coating layers is separated into two sets oflayers, and wherein a first set of layers is located on a first side ofsaid optical filter and a second set of layers is located on a secondside of said optical filter.
 11. The bar code reader of claim 6, furthercomprising an adjuster for changing the angle of incidence of the returnlight with respect to said interference-type optical filter.
 12. The barcode reader of claim 11, wherein said adjuster is passive.
 13. The barcode reader of claim 12, wherein said passive adjuster is a temperaturesensitive bimetallic spring.
 14. The bar code reader of claim 11,wherein said adjuster is an active adjuster.
 15. The bar code reader ofclaim 14, wherein said active adjuster includes: a) a sensor for sensinga temperature of said light source; and b) an actuator for positioningsaid optical filter at an angle which increases the signal-to-noiseratio of said bar code reader.
 16. The bar code reader of claim 15,wherein said active adjuster further includes a look-up table forcorrelating a set of angular positions of said optical filter to a setof temperatures of said light source.
 17. The bar code reader of claim15, wherein said active adjuster is an analog electromechanicaladjuster.
 18. The bar code reader of claim 14, wherein said activeadjuster includes: a) a sensor for sensing a wavelength of said laserbeam; b) an actuator for positioning said optical filter at an anglewhich increases the signal-to-noise ratio of said bar code reader basedon the sensed wavelength.
 19. The bar code reader of claim 18, whereinsaid active adjuster further includes a look-up table for correlating aset of angular positions of said optical filter to a set of wavelengthsof said laser beam.
 20. The bar code reader of claim 18, wherein saidactive adjuster is an analog electromechanical adjuster.
 21. The barcode reader of claim 1, wherein said tunable optical filter is anLCD-type optical filter.
 22. The bar code reader of claim 1, whereinsaid tunable optical filter is an acoustic-type optical filter.
 23. Thebar code reader of claim 1, wherein said tunable optical filter is adiffractive-type optical filter.
 24. The bar code reader of claim 1,wherein said tunable optical filter is a band-pass tunable opticalfilter and said filtering characteristic is a band-pass filteringcharacteristic.
 25. A bar code reader comprising: a) a light source forgenerating a bar code illuminating beam; b) a detector for detectingreturn light reflected from a bar code illuminated by the illuminatingbeam and producing a signal corresponding thereto; c) an optical filterfor filtering the return light before being applied to the detector; andd) a controller for adjusting the wavelength of the illuminating beam tomaximize the signal to noise ratio of the signal produced by thedetector.
 26. The bar code reader according to claim 25, wherein thecontroller comprises a modulator for modulating the temperature of thelight source.
 27. The bar code reader according to claim 25, wherein thecontroller comprises a modulator for modulating the current supplied tothe light source.
 28. A method for reading bar code comprising the stepsof: a) illuminating a bar code with a bar code illuminating beamgenerated by a light source; b) detecting return light reflected from abar code illuminated by the illuminating beam with a detector producinga signal corresponding thereto; c) filtering the return light beforebeing applied to the detector; and d) adjusting the wavelength of theilluminating beam to maximize the signal to noise ratio of the signalproduced by the detector.
 29. The method according to claim 25, whereinthe step of adjusting comprises modulating at least one of thetemperature of and current supplied to the light source.
 30. A methodfor reading bar codes including the steps of: a) generating a bar codeilluminating beam; b) illuminating a bar code with said beam; c) passinga return light reflected from the illuminated bar code through a tunableoptical filter having an adjustable filtering characteristic; d)detecting the return light passed through said tunable optical filter;and e) adjusting the filtering characteristic of said tunable opticalfilter to maximize a signal-to-noise ratio of said bar code reader. 31.A method of making an optical reader including the steps of: a)generating an illuminating beam using a light; b) illuminating a targetwith said illuminating beam; c) passing a return light reflected fromthe illuminated target through a tunable optical filter having afiltering characteristic which varies with an angle of incidence of thereturn light to said optical filter; d) detecting an intensity of thereturn light passed through said tunable optical filter; e) adjustingthe filtering characteristic of said optical filter by changing theangle of incidence of the return light in order to increase asignal-to-noise ratio of said optical reader.
 32. A data acquisitiondevice for reading optical indicia comprising: a) device housing; and b)an indicia reading module including i) a light source located withinsaid device housing for generating an indicia illuminating beam; ii) atunable optical filter located within said device housing for passing areturn light reflected from an illuminated indicia, said tunable opticalfilter having a filtering characteristic which varies with an angle ofincidence of the return light to said tunable optical filter; and iii) adetector located within said device housing for detecting the returnlight passed through said tunable optical filter; whereby said filteringcharacteristic of said tunable optical filter is adjusted by a change ofthe angle of incidence of the return light to said tunable opticalfilter.